drr-2 encodes an eIF4H that acts downstream of TOR in diet-restriction-induced longevity of C. elegans

Neuronal MML-1/MXL-2 regulates systemic aging via glutamate transporter and cell nonautonomous autophagic and peroxidase activity

Accumulating evidence has demonstrated the presence of intertissue-communication regulating systemic aging, but the underlying molecular network has not been fully explored. We and others previously showed that two basic helix-loop-helix transcription factors, MML-1 and HLH-30, are required for lifespan extension in several longevity paradigms, including germlineless Caenorhabditis elegans. However, it is unknown what tissues these factors target to promote longevity. Here, using tissue-specific knockdown experiments, we found that MML-1 and its heterodimer partners MXL-2 and HLH-30 act primarily in neurons to extend longevity in germlineless animals. Interestingly, however, the downstream cascades of MML-1 in neurons were distinct from those of HLH-30. Neuronal RNA interference (RNAi)-based transcriptome analysis revealed that the glutamate transporter GLT-5 is a downstream target of MML-1 but not HLH-30. Furthermore, the MML-1-GTL-5 axis in neurons is critical to prevent an age-dependent collapse of proteostasis and increased oxidative stress through autophagy and peroxidase MLT-7, respectively, in long-lived animals. Collectively, our study revealed that systemic aging is regulated by a molecular network involving neuronal MML-1 function in both neural and peripheral tissues.

Cytoplasmic and mitochondrial aminoacyl-tRNA synthetases differentially regulate lifespan in Caenorhabditis elegans

Reducing the rate of translation promotes longevity in multiple organisms, representing a conserved mechanism for lifespan extension. Aminoacyl-tRNA synthetases (ARSs) catalyze the loading of amino acids to their cognate tRNAs, thereby playing an essential role in translation. Mutations in ARS genes are associated with various human diseases. However, little is known about the role of ARSs in aging, particularly whether and how these genes regulate lifespan. Here, using Caenorhabditis elegans as a model, we systematically characterized the role of all three types of ARS genes in lifespan regulation, including mitochondrial, cytoplasmic, and cyto-mito bifunctional ARS genes. We found that, as expected, RNAi knockdown of mitochondrial ARS genes extended lifespan. Surprisingly, knocking down cytoplasmic or cyto-mito bifunctional ARS genes shortened lifespan, though such treatment reduced the rate of translation. These results reveal opposing roles of mitochondrial and cytoplasmic ARSs in lifespan regulation, demonstrating that inhibiting translation may not always extend lifespan.

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RNAi knockdown of mitochondrial ARS genes extends lifespan via UPR^mt

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RNAi knockdown of cytoplasmic or cyto-mito bifunctional ARS genes shortens lifespan

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Inhibiting translation may not always extend lifespan

Molecular mechanisms of exceptional lifespan increase of Drosophila melanogaster with different genotypes after combinations of pro-longevity interventions

Aging is one of the global challenges of our time. The search for new anti-aging interventions is also an issue of great actuality. We report on the success of Drosophila melanogaster lifespan extension under the combined influence of dietary restriction, co-administration of berberine, fucoxanthin, and rapamycin, photodeprivation, and low-temperature conditions up to 185 days in w¹¹¹⁸ strain and up to 213 days in long-lived E(z)/w mutants. The trade-off was found between longevity and locomotion. The transcriptome analysis showed an impact of epigenetic alterations, lipid metabolism, cellular respiration, nutrient sensing, immune response, and autophagy in the registered effect.

The lifespan of fruit flies can be extended up to 213 days under specialized conditions.

Targeting metabolic pathways for extension of lifespan and healthspan across multiple species

Metabolism plays a significant role in the regulation of aging at different levels, and metabolic reprogramming represents a major driving force in aging. Metabolic reprogramming leads to impaired organismal fitness, an age-dependent increase in susceptibility to diseases, decreased ability to mount a stress response, and increased frailty. The complexity of age-dependent metabolic reprogramming comes from the multitude of levels on which metabolic changes can be connected to aging and regulation of lifespan. This is further complicated by the different metabolic requirements of various tissues, cross-organ communication via metabolite secretion, and direct effects of metabolites on epigenetic state and redox regulation; however, not all of these changes are causative to aging. Studies in yeast, flies, worms, and mice have played a crucial role in identifying mechanistic links between observed changes in various metabolic traits and their effects on lifespan. Here, we review how changes in the organismal and organ-specific metabolome are associated with aging and how targeting of any one of over a hundred different targets in specific metabolic pathways can extend lifespan. An important corollary is that restriction or supplementation of different metabolites can change activity of these metabolic pathways in ways that improve healthspan and extend lifespan in different organisms. Due to the high levels of conservation of metabolism in general, translating findings from model systems to human beings will allow for the development of effective strategies for human health- and lifespan extension.

S-adenosyl methionine synthetase SAMS-5 mediates dietary restriction-induced longevity in Caenorhabditis elegans

S-adenosyl methionine synthetase (SAMS) catalyzes the biosynthesis of S-adenosyl methionine (SAM), which serves as a universal methyl group donor for numerous biochemical reactions. Previous studies have clearly demonstrated that SAMS-1, a C. elegans homolog of mammalian SAMS, is critical for dietary restriction (DR)-induced longevity in Caenorhabditis elegans. In addition to SAMS-1, three other SAMS paralogs have been identified in C. elegans. However, their roles in longevity regulation have never been explored. Here, we show that depletion of sams-5, but not sams-3 or sams-4, can extend lifespan in worms. However, the phenotypes and expression pattern of sams-5 are distinct from sams-1, suggesting that these two SAMSs might regulate DR-induced longevity via different mechanisms. Through the genetic epistasis analysis, we have identified that sams-5 is required for DR-induced longevity in a pha-4/FOXA dependent manner.

Methionine metabolism and methyltransferases in the regulation of aging and lifespan extension across species

Methionine restriction (MetR) extends lifespan across different species and exerts beneficial effects on metabolic health and inflammatory responses. In contrast, certain cancer cells exhibit methionine auxotrophy that can be exploited for therapeutic treatment, as decreasing dietary methionine selectively suppresses tumor growth. Thus, MetR represents an intervention that can extend lifespan with a complementary effect of delaying tumor growth. Beyond its function in protein synthesis, methionine feeds into complex metabolic pathways including the methionine cycle, the transsulfuration pathway, and polyamine biosynthesis. Manipulation of each of these branches extends lifespan; however, the interplay between MetR and these branches during regulation of lifespan is not well understood. In addition, a potential mechanism linking the activity of methionine metabolism and lifespan is regulation of production of the methyl donor S-adenosylmethionine, which, after transferring its methyl group, is converted to S-adenosylhomocysteine. Methylation regulates a wide range of processes, including those thought to be responsible for lifespan extension by MetR. Although the exact mechanisms of lifespan extension by MetR or methionine metabolism reprogramming are unknown, it may act via reducing the rate of translation, modifying gene expression, inducing a hormetic response, modulating autophagy, or inducing mitochondrial function, antioxidant defense, or other metabolic processes. Here, we review the mechanisms of lifespan extension by MetR and different branches of methionine metabolism in different species and the potential for exploiting the regulation of methyltransferases to delay aging.

TOR Signaling in Caenorhabditis elegans Development, Metabolism, and Aging

The Target of Rapamycin (TOR or mTOR) is a serine/threonine kinase that regulates growth, development, and behaviors by modulating protein synthesis, autophagy, and multiple other cellular processes in response to changes in nutrients and other cues. Over recent years, TOR has been studied intensively in mammalian cell culture and genetic systems because of its importance in growth, metabolism, cancer, and aging. Through its advantages for unbiased, and high-throughput, genetic and in vivo studies, Caenorhabditis elegans has made major contributions to our understanding of TOR biology. Genetic analyses in the worm have revealed unexpected aspects of TOR functions and regulation, and have the potential to further expand our understanding of how growth and metabolic regulation influence development. In the aging field, C. elegans has played a leading role in revealing the promise of TOR inhibition as a strategy for extending life span, and identifying mechanisms that function upstream and downstream of TOR to influence aging. Here, we review the state of the TOR field in C. elegans, and focus on what we have learned about its functions in development, metabolism, and aging. We discuss knowledge gaps, including the potential pitfalls in translating findings back and forth across organisms, but also describe how TOR is important for C. elegans biology, and how C. elegans work has developed paradigms of great importance for the broader TOR field.

Long-lived Temnothorax ant queens switch from investment in immunity to antioxidant production with age

Senescence is manifested by an increase in molecular damage and a deterioration of biological functions with age. In most organisms, body maintenance is traded-off with reproduction. This negative relationship between longevity and fecundity is also evident on the molecular level. Exempt from this negative trait association, social insect queens are both extremely long-lived and highly fecund. Here, we study changes in gene expression with age and fecundity in ant queens to understand the molecular basis of their long lifespan. We analyse tissue-specific gene expression in young founding queens and old fecund queens of the ant Temnothorax rugatulus. More genes altered their expression with age in the fat body than in the brain. Despite strong differences in ovary development, few fecundity genes were differentially expressed. Young founding queens invested in immunity (i.e. activation of Toll signalling pathway) and resistance against environmental and physiological stress (i.e. down-regulation of TOR pathway). Conversely, established older queens invested into anti-aging mechanisms through an overproduction of antioxidants (i.e. upregulation of catalase, superoxide dismutase). Finally, we identified candidate genes and pathways, potentially involved in the association between fertility and longevity in social insects and its proximate basis.

A salicylic acid derivative extends the lifespan of Caenorhabditis elegans by activating autophagy and the mitochondrial unfolded protein response

Plant extracts containing salicylates are probably the most ancient remedies to reduce fever and ease aches of all kind. Recently, it has been shown that salicylates activate adenosine monophosphate‐activated kinase (AMPK), which is now considered as a promising target to slow down aging and prevent age‐related diseases in humans. Beneficial effects of AMPK activation on lifespan have been discovered in the model organism Caenorhabditis elegans (C. elegans). Indeed, salicylic acid and acetylsalicylic acid extend lifespan in worms by activating AMPK and the forkhead transcription factor DAF‐16/FOXO. Here, we investigated whether another salicylic acid derivative 5‐octanoyl salicylic acid (C8‐SA), developed as a controlled skin exfoliating ingredient, had similar properties using C. elegans as a model. We show that C8‐SA increases lifespan of C. elegans and that a variety of pathways and genes are required for C8‐SA‐mediated lifespan extension. C8‐SA activates AMPK and inhibits TOR both in nematodes and in primary human keratinocytes. We also show that C8‐SA can induce both autophagy and the mitochondrial unfolded protein response (UPR^mit) in nematodes. This induction of both processes is fully required for lifespan extension in the worm. In addition, we found that the activation of autophagy by C8‐SA fails to occur in worms with compromised UPR^mit, suggesting a mechanistic link between these two processes. Mutants that are defective in the mitochondrial unfolded protein response exhibit constitutive high autophagy levels. Taken together, these data therefore suggest that C8‐SA positively impacts longevity in worms through induction of autophagy and the UPR^mit.

Emerging topics in C. elegans aging research: Transcriptional regulation, stress response and epigenetics

Key discoveries in aging research have been made possible with the use of model organisms. Caenorhabditis elegans is a short-lived nematode that has become a well-established system to study aging. The practicality and powerful genetic manipulations associated with this metazoan have revolutionized our ability to understand how organisms age. 25 years after the publication of the discovery of the daf-2 gene as a genetic modifier of lifespan, C. elegans remains as relevant as ever in the quest to understand the process of aging. Nematode aging research has proven useful in identifying transcriptional regulators, small molecule signals, cellular mechanisms, epigenetic modifications associated with stress resistance and longevity, and lifespan-extending compounds. Here, we review recent discoveries and selected topics that have emerged in aging research using this incredible little worm.

Nucleolar Function in Lifespan Regulation

The nucleolus is a prominent membraneless organelle residing within the nucleus. The nucleolus has been regarded as a housekeeping structure mainly known for its role in ribosomal RNA (rRNA) production and ribosome assembly. However, accumulating evidence has revealed its functions in numerous cellular processes that control organismal physiology, thereby taking the nucleolus much beyond its conventional role in ribosome biogenesis. Perturbations in nucleolar functions have been associated with severe diseases such as cancer and progeria. Recent studies have also uncovered the role of the nucleolus in development and aging. In this review we discuss major nucleolar functions that impact organismal aging.

Dietary restriction and lifespan: Lessons from invertebrate models

Dietary restriction (DR) is the most robust environmental manipulation known to increase active and healthy lifespan in many species. Despite differences in the protocols and the way DR is carried out in different organisms, conserved relationships are emerging among multiple species. Elegant studies from numerous model organisms are further defining the importance of various nutrient-signaling pathways including mTOR (mechanistic target of rapamycin), insulin/IGF-1-like signaling and sirtuins in mediating the effects of DR. We here review current advances in our understanding of the molecular mechanisms altered by DR to promote lifespan in three major invertebrate models, the budding yeast Saccharomyces cerevisiae, the nematode Caenorhabditis elegans, and the fruit fly Drosophila melanogaster.

Quantification of Information Encoded by Gene Expression Levels During Lifespan Modulation Under Broad-range Dietary Restriction in C. elegans

Sensory systems allow animals to detect, process, and respond to their environment. Food abundance is an environmental cue that has profound effects on animal physiology and behavior. Recently, we showed that modulation of longevity in the nematode Caenorhabditis elegans by food abundance is more complex than previously recognized. The responsiveness of the lifespan to changes in food level is determined by specific genes that act by controlling information processing within a neural circuit. Our framework combines genetic analysis, high-throughput quantitative imaging and information theory. Here, we describe how these techniques can be used to characterize any gene that has a physiological relevance to broad-range dietary restriction. Specifically, this workflow is designed to reveal how a gene of interest regulates lifespan under broad-range dietary restriction; then to establish how the expression of the gene varies with food level; and finally, to provide an unbiased quantification of the amount of information conveyed by gene expression about food abundance in the environment. When several genes are examined simultaneously under the context of a neural circuit, this workflow can uncover the coding strategy employed by the circuit.

Pleiotropic responses to methionine restriction

Methionine restriction (MR) extends lifespan across different species. The main responses of rodent models to MR are well-documented in adipose tissue (AT) and liver, which have reduced mass and improved insulin sensitivity, respectively. Recently, molecular mechanisms that improve healthspan have been identified in both organs during MR. In fat, MR induced a futile lipid cycle concomitant with beige AT accumulation, producing elevated energy expenditure. In liver, MR upregulated fibroblast growth factor 21 and improved glucose metabolism in aged mice and in response to a high-fat diet. Furthermore, MR also reduces mitochondrial oxidative stress in various organs such as liver, heart, kidneys, and brain. Other effects of MR have also been reported in such areas as cardiac function in response to hyperhomocysteinemia (HHcy), identification of molecular mechanisms in bone development, and enhanced epithelial tight junction. In addition, rodent models of cancer responded positively to MR, as has been reported in colon, prostate, and breast cancer studies. The beneficial effects of MR have also been documented in a number of invertebrate model organisms, including yeast, nematodes, and fruit flies. MR not only promotes extended longevity in these organisms, but in the case of yeast has also been shown to improve stress tolerance. In addition, expression analyses of yeast and Drosophila undergoing MR have identified multiple candidate mediators of the beneficial effects of MR in these models. In this review, we emphasize other in vivo effects of MR such as in cardiovascular function, bone development, epithelial tight junction, and cancer. We also discuss the effects of MR in invertebrates.

Comparative idiosyncrasies in life extension by reduced mTOR signalling and its distinctiveness from dietary restriction

Reduced mechanistic target of rapamycin (mTOR) signalling extends lifespan in yeast, nematodes, fruit flies and mice, highlighting a physiological pathway that could modulate aging in evolutionarily divergent organisms. This signalling system is also hypothesized to play a central role in lifespan extension via dietary restriction. By collating data from 48 available published studies examining lifespan with reduced mTOR signalling, we show that reduced mTOR signalling provides similar increases in median lifespan across species, with genetic mTOR manipulations consistently providing greater life extension than pharmacological treatment with rapamycin. In contrast to the consistency in changes in median lifespan, however, the demographic causes for life extension are highly species specific. Reduced mTOR signalling extends lifespan in nematodes by strongly reducing the degree to which mortality rates increase with age (aging rate). By contrast, life extension in mice and yeast occurs largely by pushing back the onset of aging, but not altering the shape of the mortality curve once aging starts. Importantly, in mice, the altered pattern of mortality induced by reduced mTOR signalling is different to that induced by dietary restriction, which reduces the rate of aging. Effects of mTOR signalling were also sex dependent, but only within mice, and not within flies, thus again species specific. An alleviation of age‐associated mortality is not a shared feature of reduced mTOR signalling across model organisms and does not replicate the established age‐related survival benefits of dietary restriction.

Endurance exercise and selective breeding for longevity extend Drosophila healthspan by overlapping mechanisms

Endurance exercise has emerged as a powerful intervention that promotes healthy aging by maintaining the functional capacity of critical organ systems. In addition, long-term exercise reduces the incidence of age-related diseases in humans and in model organisms. Despite these evident benefits, the genetic pathways required for exercise interventions to achieve these effects are still relatively poorly understood. Here, we compare gene expression changes during endurance training in Drosophila melanogaster to gene expression changes during selective breeding for longevity. Microarrays indicate that 65% of gene expression changes found in flies selectively bred for longevity are also found in flies subjected to three weeks of exercise training. We find that both selective breeding and endurance training increase endurance, cardiac performance, running speed, flying height, and levels of autophagy in adipose tissue. Both interventions generally upregulate stress defense, folate metabolism, and lipase activity, while downregulating carbohydrate metabolism and odorant receptor expression. Several members of the methuselah-like (mthl) gene family are downregulated by both interventions. Knockdown of mthl-3 was sufficient to provide extension of negative geotaxis behavior, endurance and cardiac stress resistance. These results provide support for endurance exercise as a broadly acting anti-aging intervention and confirm that exercise training acts in part by targeting longevity assurance pathways.

Reciprocal Changes in Phosphoenolpyruvate Carboxykinase and Pyruvate Kinase with Age Are a Determinant of Aging in Caenorhabditis elegans

Aging involves progressive loss of cellular function and integrity, presumably caused by accumulated stochastic damage to cells. Alterations in energy metabolism contribute to aging, but how energy metabolism changes with age, how these changes affect aging, and whether they can be modified to modulate aging remain unclear. In locomotory muscle of post-fertile Caenorhabditis elegans, we identified a progressive decrease in cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C), a longevity-associated metabolic enzyme, and a reciprocal increase in glycolytic pyruvate kinase (PK) that were necessary and sufficient to limit lifespan. Decline in PEPCK-C with age also led to loss of cellular function and integrity including muscle activity, and cellular senescence. Genetic and pharmacologic interventions of PEPCK-C, muscle activity, and AMPK signaling demonstrate that declines in PEPCK-C and muscle function with age interacted to limit reproductive life and lifespan via disrupted energy homeostasis. Quantifications of metabolic flux show that reciprocal changes in PEPCK-C and PK with age shunted energy metabolism toward glycolysis, reducing mitochondrial bioenergetics. Last, calorie restriction countered changes in PEPCK-C and PK with age to elicit anti-aging effects via TOR inhibition. Thus, a programmed metabolic event involving PEPCK-C and PK is a determinant of aging that can be modified to modulate aging.

Metabolome and proteome changes with aging in Caenorhabditis elegans

To expand the understanding of aging in the model organism Caenorhabditis elegans, global quantification of metabolite and protein levels in young and aged nematodes was performed using mass spectrometry. With age, there was a decreased abundance of proteins functioning in transcription termination, mRNA degradation, mRNA stability, protein synthesis, and proteasomal function. Furthermore, there was altered S-adenosyl methionine metabolism as well as a decreased abundance of the S-adenosyl methionine synthetase (SAMS-1) protein. Other aging-related changes included alterations in free fatty acid levels and composition, decreased levels of ribosomal proteins, decreased levels of NADP-dependent isocitrate dehydrogenase (IDH1), a shift in the cellular redox state, an increase in sorbitol content, alterations in free amino acid levels, and indications of altered muscle function and sarcoplasmic reticulum Ca(2+) homeostasis. There were also decreases in pyrimidine and purine metabolite levels, most markedly nitrogenous bases. Supplementing the culture medium with cytidine (a pyrimidine nucleoside) or hypoxanthine (a purine base) increased lifespan slightly, suggesting that aging-induced alterations in ribonucleotide metabolism affect lifespan. An age-related increase in body size, lipotoxicity from ectopic yolk lipoprotein accumulation, a decline in NAD(+) levels, and mitochondrial electron transport chain dysfunction may explain many of these changes. In addition, dietary restriction in aged worms resulting from sarcopenia of the pharyngeal pump likely decreases the abundance of SAMS-1, possibly leading to decreased phosphatidylcholine levels, larger lipid droplets, and ER and mitochondrial stress. The complementary use of proteomics and metabolomics yielded unique insights into the molecular processes altered with age in C. elegans.

Enhancing S-adenosyl-methionine catabolism extends Drosophila lifespan

Methionine restriction extends the lifespan of various model organisms. Limiting S-adenosyl-methionine (SAM) synthesis, the first metabolic reaction of dietary methionine, extends longevity in Caenorhabditis elegans but accelerates pathology in mammals. Here, we show that, as an alternative to inhibiting SAM synthesis, enhancement of SAM catabolism by glycine N-methyltransferase (Gnmt) extends the lifespan in Drosophila. Gnmt strongly buffers systemic SAM levels by producing sarcosine in either high-methionine or low-sams conditions. During ageing, systemic SAM levels in flies are increased. Gnmt is transcriptionally induced in a dFoxO-dependent manner; however, this is insufficient to suppress SAM elevation completely in old flies. Overexpression of gnmt suppresses this age-dependent SAM increase and extends longevity. Pro-longevity regimens, such as dietary restriction or reduced insulin signalling, attenuate the age-dependent SAM increase, and rely at least partially on Gnmt function to exert their lifespan-extending effect in Drosophila. Our study suggests that regulation of SAM levels by Gnmt is a key component of lifespan extension.

Inhibiting the formation of S-adenosyl-methionine (SAM) increases worm but not fly lifespan. Here the authors show that humans and flies possess the SAM-consuming enzyme Gnmt, the activity of which is regulated by lifespan-extending interventions, and that knockdown of Gnmt extends fly lifespan.

Protein synthesis as an integral quality control mechanism during ageing

Ageing is manifested as functional and structural deterioration that affects cell and tissue physiology. mRNA translation is a central cellular process, supplying cells with newly synthesized proteins. Accumulating evidence suggests that alterations in protein synthesis are not merely a corollary but rather a critical factor for the progression of ageing. Here, we survey protein synthesis regulatory mechanisms and focus on the pre-translational regulation of the process exerted by non-coding RNA species, RNA binding proteins and alterations of intrinsic RNA properties. In addition, we discuss the tight relationship between mRNA translation and two central pathways that modulate ageing, namely the insulin/IGF-1 and TOR signalling cascades. A thorough understanding of the complex interplay between protein synthesis regulation and ageing will provide critical insights into the pathogenesis of age-related disorders, associated with impaired proteostasis and protein quality control.

A gene-expression-based neural code for food abundance that modulates lifespan

How the nervous system internally represents environmental food availability is poorly understood. Here, we show that quantitative information about food abundance is encoded by combinatorial neuron-specific gene-expression of conserved TGFβ and serotonin pathway components in Caenorhabditis elegans. Crosstalk and auto-regulation between these pathways alters the shape, dynamic range, and population variance of the gene-expression responses of daf-7 (TGFβ) and tph-1 (tryptophan hydroxylase) to food availability. These intricate regulatory features provide distinct mechanisms for TGFβ and serotonin signaling to tune the accuracy of this multi-neuron code: daf-7 primarily regulates gene-expression variability, while tph-1 primarily regulates the dynamic range of gene-expression responses. This code is functional because daf-7 and tph-1 mutations bidirectionally attenuate food level-dependent changes in lifespan. Our results reveal a neural code for food abundance and demonstrate that gene expression serves as an additional layer of information processing in the nervous system to control long-term physiology.

DOI:http://dx.doi.org/10.7554/eLife.06259.001

To maximize their chances of survival, animals need to be able to sense changes in the abundance of food in their environment and respond in an appropriate manner. The nervous system is able to sense cues from the environment and coordinate responses in the whole organism, but it is not clear how this leads to long-term changes in the organism's biology.

In nematode worms, two genes called daf-7 and tph-1 appear to be involved in connecting the sensing of food availability with changes in the biology of the organism. The daf-7 gene encodes a hormone, while tph-1 encodes an enzyme that makes a neurochemical called serotonin.

Here, Entchev, Patel, Zhan et al. found that daf-7 and tph-1 genes are active in three pairs of neurons in nematode worms. The experiments show that these neurons collectively form a circuit that carries information about the abundance of food, which leads to changes in how long the worms live. When this circuit was disrupted by removing these genes, the worms' ability to adjust their lifespan in response to changes in the availability of food was weakened, likely because they were unable to sense food. The experiments also show that the circuit regulates itself, largely because daf-7 and tph-1 are able to control each-other's activity.

Together, these results suggest that changing the activity of certain genes in neurons enables nematode worms to alter their biology in response to changes in the availability of food. Neurons in the brain use electrical activity to communicate and process information and Entchev, Patel, Zhan et al.'s findings imply that gene activity can also perform a similar role.

DOI:http://dx.doi.org/10.7554/eLife.06259.002

Monitoring newly synthesized proteins over the adult life span of Caenorhabditis elegans

Little is known regarding how the synthesis and degradation of individual proteins change during the life of an organism. Such knowledge is vital to understanding the aging process. To fill this knowledge gap, we monitored newly synthesized proteins on a proteome scale in Caenorhabditis elegans over time during adulthood using a stable-isotope labeling by amino acids in cell culture (SILAC)-based label-chase approach. For most proteins, the rate of appearance of newly synthesized protein was high during the first 5 days of adulthood, slowed down between the fifth and the 11th days, and then increased again after the 11th day. However, the magnitude of appearance rate differed significantly from protein to protein. For example, the appearance of newly synthesized protein was fast for proteins involved in embryonic development, transcription regulation, and lipid binding/transport, with >70% of these proteins newly synthesized by day 5 of adulthood, whereas it was slow for proteins involved in cellular assembly and motility, such as actin and myosin, with <70% of these proteins newly synthesized even on day 16. The late-life increase of newly synthesized protein was especially high for ribosomal proteins and ATP synthases. We also investigated the effect of RNAi-mediated knockdown of the rpl-9 (ribosomal protein), atp-3 (ATP synthase), and ril-1 (RNAi-induced longevity-1) genes and found that inhibiting the expression of atp-3 and ril-1 beginning in late adulthood is still effective to extend the life span of C. elegans.

Rapamycin and ageing: when, for how long, and how much?

The drug rapamycin is the only pharmacological agent thus far shown to reproducibly extend lifespan and delay a subset of age-associated pathologies in multiple strains of mice. Unfortunately, the vast majority of aging-related studies on rapamycin in mice have been performed at a single dose of the drug delivered in encapsulated form through the diet. Recently, the National Institute on Aging Interventions Testing Program reported that a three-fold higher dose of dietary rapamycin results in a significantly greater increase in lifespan. This observation demonstrates that current studies of the effects of rapamycin on lifespan and healthspan in mice are being performed under conditions that are sub-optimal. Here I argue that the failure to properly determine the dose and timing response profile for rapamycin with respect to healthy aging represents a major barrier for the field. This barrier continues to hamper our ability to gain mechanistic insights and may threaten efforts to translate these findings into interventions that promote healthy aging in people.

DAF-16 target identification in C. elegans: past, present and future

In C. elegans, mutations in the conserved insulin/IGF-1 signaling (IIS) pathway lead to a robust extension in lifespan, improved late life health, and protection from age-related disease. These effects are mediated by the FoxO transcription factor DAF-16 which lies downstream of the IIS kinase cascade. Identifying and functionally testing DAF-16 target genes has been a focal point of ageing research for the last 10 years. Here, I review the recent advances in identifying and understanding IIS/DAF-16 targets. These studies continue to reveal the intricate nature of the IIS/DAF-16 gene regulation network and are helping us to understand the mechanisms that control lifespan. Ageing and age related disease is an area of intense public interest, and the biochemical characterization of the genes involved will be critical for identifying drugs to improve the health of our ageing population.

Comparison of the mammalian insulin signalling pathway to invertebrates in the context of FOXO-mediated ageing

MOTIVATION

A large number of experimental studies on ageing focus on the effects of genetic perturbations of the insulin/insulin-like growth factor signalling pathway (IIS) on lifespan. Short-lived invertebrate laboratory model organisms are extensively used to quickly identify ageing-related genes and pathways. It is important to extrapolate this knowledge to longer lived mammalian organisms, such as mouse and eventually human, where such analyses are difficult or impossible to perform. Computational tools are needed to integrate and manipulate pathway knowledge in different species.

RESULTS

We performed a literature review and curation of the IIS and target of rapamycin signalling pathways in Mus Musculus. We compare this pathway model to the equivalent models in Drosophila melanogaster and Caenorhabtitis elegans. Although generally well-conserved, they exhibit important differences. In general, the worm and mouse pathways include a larger number of feedback loops and interactions than the fly. We identify 'functional orthologues' that share similar molecular interactions, but have moderate sequence similarity. Finally, we incorporate the mouse model into the web-service NetEffects and perform in silico gene perturbations of IIS components and analyses of experimental results. We identify sub-paths that, given a mutation in an IIS component, could potentially antagonize the primary effects on ageing via FOXO in mouse and via SKN-1 in worm. Finally, we explore the effects of FOXO knockouts in three different mouse tissues.

AVAILABILITY AND IMPLEMENTATION

http://www.ebi.ac.uk/thornton-srv/software/NetEffects.

Intestinal amino acid availability via PEPT-1 affects TORC1/2 signaling and the unfolded protein response

The intestinal peptide transporter PEPT-1 plays an important role in development, growth, reproduction, and stress tolerance in Caenorhabditis elegans, as revealed by the severe phenotype of the pept-1-deficient strain. The reduced number of offspring and increased stress resistance were shown to result from changes in the insulin/IGF-signaling cascade. To further elucidate the regulatory network behind the phenotypic alterations in PEPT1-deficient animals, a quantitative proteome analysis combined with transcriptome profiling was applied. Various target genes of XBP-1, the major mediator of the unfolded protein response, were found to be downregulated at the mRNA and protein levels, accompanied by a reduction of spliced xbp-1 mRNA. Proteome analysis also revealed a markedly reduced content of numerous ribosomal proteins. This was associated with a reduction in the protein synthesis rate in pept-1 C. elegans, a process that is strictly regulated by the TOR (target of rapamycine) complex, the cellular sensor for free amino acids. These data argue for a central role of PEPT-1 in cellular amino acid homeostasis. In PEPT-1 deficiency, amino acid levels dropped systematically, leading to alterations in protein synthesis and in the IRE-1/XBP-1 pathway.

LC-MS proteomics analysis of the insulin/IGF-1-deficient Caenorhabditis elegans daf-2(e1370) mutant reveals extensive restructuring of intermediary metabolism

The insulin/IGF-1 receptor is a major known determinant of dauer formation, stress resistance, longevity, and metabolism in Caenorhabditis elegans. In the past, whole-genome transcript profiling was used extensively to study differential gene expression in response to reduced insulin/IGF-1 signaling, including the expression levels of metabolism-associated genes. Taking advantage of the recent developments in quantitative liquid chromatography mass spectrometry (LC-MS)-based proteomics, we profiled the proteomic changes that occur in response to activation of the DAF-16 transcription factor in the germline-less glp-4(bn2);daf-2(e1370) receptor mutant. Strikingly, the daf-2 profile suggests extensive reorganization of intermediary metabolism, characterized by the upregulation of many core intermediary metabolic pathways. These include glycolysis/gluconeogenesis, glycogenesis, pentose phosphate cycle, citric acid cycle, glyoxylate shunt, fatty acid β-oxidation, one-carbon metabolism, propionate and tyrosine catabolism, and complexes I, II, III, and V of the electron transport chain. Interestingly, we found simultaneous activation of reciprocally regulated metabolic pathways, which is indicative of spatiotemporal coordination of energy metabolism and/or extensive post-translational regulation of these enzymes. This restructuring of daf-2 metabolism is reminiscent to that of hypometabolic dauers, allowing the efficient and economical utilization of internal nutrient reserves and possibly also shunting metabolites through alternative energy-generating pathways to sustain longevity.

Cellular and molecular longevity pathways: the old and the new

Human lifespan has been increasing steadily during modern times, mainly due to medical advancements that combat infant mortality and various life-threatening diseases. However, this gratifying longevity rise is accompanied by growing incidences of devastating age-related pathologies. Understanding the cellular and molecular mechanisms that underlie aging and regulate longevity is of utmost relevance towards offsetting the impact of age-associated disorders and increasing the quality of life for the elderly. Several evolutionarily conserved pathways that modulate lifespan have been identified in organisms ranging from yeast to primates. Here we survey recent findings highlighting the interplay of various genetic, epigenetic, and cell-specific factors, and also symbiotic relationships, as longevity determinants. We further discuss outstanding matters within the framework of emerging, integrative views of aging.

S-Adenosyl methionine synthetase 1 limits fat storage in Caenorhabditis elegans

Cytosolic lipid droplets are versatile, evolutionarily conserved organelles that are important for the storage and utilization of lipids in almost all cell types. To obtain insight into the physiological importance of lipid droplet size, we isolated and characterized a new S-adenosyl methionine synthetase 1 (SAMS-1)-deficient Caenorhabditis elegans mutant, which have enlarged lipid droplets throughout its life cycle. We found that the sams-1 mutant showed a markedly reduced body size and progeny number; impaired synthesis of phosphatidylcholine, a major membrane phospholipid; and elevated expression of key lipogenic genes, such as dgat-2, resulting in the accumulation of triacylglyceride in fewer, but larger, lipid droplets. The sams-1 mutant store more than 50 % (wild type: 10 %) of its intestinal fat in large lipid droplets, ≥10 μm(3) in size. In response to starvation, SAMS-1 deficiency causes reduced depletion of a subset of lipid droplets located in the anterior intestine. Given the importance of liberation of fatty acids from lipid droplets, we propose that the physiological function of SAMS-1, a highly conserved enzyme involved in one-carbon metabolism, is the limitation of fat storage to ensure proper growth and reproduction.

Reduced insulin/insulin-like growth factor-1 signaling and dietary restriction inhibit translation but preserve muscle mass in Caenorhabditis elegans

Reduced signaling through the C. elegans insulin/insulin-like growth factor-1-like tyrosine kinase receptor daf-2 and dietary restriction via bacterial dilution are two well-characterized lifespan-extending interventions that operate in parallel or through (partially) independent mechanisms. Using accurate mass and time tag LC-MS/MS quantitative proteomics, we detected that the abundance of a large number of ribosomal subunits is decreased in response to dietary restriction, as well as in the daf-2(e1370) insulin/insulin-like growth factor-1-receptor mutant. In addition, general protein synthesis levels in these long-lived worms are repressed. Surprisingly, ribosomal transcript levels were not correlated to actual protein abundance, suggesting that post-transcriptional regulation determines ribosome content. Proteomics also revealed the increased presence of many structural muscle cell components in long-lived worms, which appeared to result from the prioritized preservation of muscle cell volume in nutrient-poor conditions or low insulin-like signaling. Activation of DAF-16, but not diet restriction, stimulates mRNA expression of muscle-related genes to prevent muscle atrophy. Important daf-2-specific proteome changes include overexpression of aerobic metabolism enzymes and general activation of stress-responsive and immune defense systems, whereas the increased abundance of many protein subunits of the proteasome core complex is a dietary-restriction-specific characteristic.

A cytoprotective perspective on longevity regulation

There are many mechanisms of lifespan extension, including the disruption of insulin/insulin-like growth factor 1 (IGF-1) signaling, metabolism, translation, and feeding. Despite the disparate functions of these pathways, inhibition of each induces responses that buffer stress and damage. Here, emphasizing data from genetic analyses in Caenorhabditis elegans, we explore the effectors and upstream regulatory components of numerous cytoprotective mechanisms activated as major elements of longevity programs, including detoxification, innate immunity, proteostasis, and oxidative stress response. We show that their induction underpins longevity extension across functionally diverse triggers and across species. Intertwined with the evolution of longevity, cytoprotective pathways are coupled to the surveillance of core cellular components, with important implications in normal and aberrant responses to drugs, chemicals, and pathogens.

Metformin retards aging in C. elegans by altering microbial folate and methionine metabolism

The biguanide drug metformin is widely prescribed to treat type 2 diabetes and metabolic syndrome, but its mode of action remains uncertain. Metformin also increases lifespan in Caenorhabditis elegans cocultured with Escherichia coli. This bacterium exerts complex nutritional and pathogenic effects on its nematode predator/host that impact health and aging. We report that metformin increases lifespan by altering microbial folate and methionine metabolism. Alterations in metformin-induced longevity by mutation of worm methionine synthase (metr-1) and S-adenosylmethionine synthase (sams-1) imply metformin-induced methionine restriction in the host, consistent with action of this drug as a dietary restriction mimetic. Metformin increases or decreases worm lifespan, depending on E. coli strain metformin sensitivity and glucose concentration. In mammals, the intestinal microbiome influences host metabolism, including development of metabolic disease. Thus, metformin-induced alteration of microbial metabolism could contribute to therapeutic efficacy—and also to its side effects, which include folate deficiency and gastrointestinal upset.

PaperClip

A novel sphingolipid-TORC1 pathway critically promotes postembryonic development in Caenorhabditis elegans

Regulation of animal development in response to nutritional cues is an intensely studied problem related to disease and aging. While extensive studies indicated roles of the Target of Rapamycin (TOR) in sensing certain nutrients for controlling growth and metabolism, the roles of fatty acids and lipids in TOR-involved nutrient/food responses are obscure. Caenorhabditis elegans halts postembryonic growth and development shortly after hatching in response to monomethyl branched-chain fatty acid (mmBCFA) deficiency. Here, we report that an mmBCFA-derived sphingolipid, d17iso-glucosylceramide, is a critical metabolite in regulating growth and development. Further analysis indicated that this lipid function is mediated by TORC1 and antagonized by the NPRL-2/3 complex in the intestine. Strikingly, the essential lipid function is bypassed by activating TORC1 or inhibiting NPRL-2/3. Our findings uncover a novel lipid-TORC1 signaling pathway that coordinates nutrient and metabolic status with growth and development, advancing our understanding of the physiological roles of mmBCFAs, ceramides, and TOR. DOI:http://dx.doi.org/10.7554/eLife.00429.001.

Mitochondria and organismal longevity

Mitochondria are essential for various biological processes including cellular energy production. The oxidative stress theory of aging proposes that mitochondria play key roles in aging by generating reactive oxygen species (ROS), which indiscriminately damage macromolecules and lead to an age-dependent decline in biological function. However, recent studies show that increased levels of ROS or inhibition of mitochondrial function can actually delay aging and increase lifespan. The aim of this review is to summarize recent findings regarding the role of mitochondria in organismal aging processes. We will discuss how mitochondria contribute to evolutionarily conserved longevity pathways, including mild inhibition of respiration, dietary restriction, and target of rapamycin (TOR) signaling.

mTOR Inhibition: From Aging to Autism and Beyond

The mechanistic target of rapamycin (mTOR) is a highly conserved protein that regulates growth and proliferation in response to environmental and hormonal cues. Broadly speaking, organisms are constantly faced with the challenge of interpreting their environment and making a decision between "grow or do not grow." mTOR is a major component of the network that makes this decision at the cellular level and, to some extent, the tissue and organismal level as well. Although overly simplistic, this framework can be useful when considering the myriad functions ascribed to mTOR and the pleiotropic phenotypes associated with genetic or pharmacological modulation of mTOR signaling. In this review, I will consider mTOR function in this context and attempt to summarize and interpret the growing body of literature demonstrating interesting and varied effects of mTOR inhibitors. These include robust effects on a multitude of age-related parameters and pathologies, as well as several other processes not obviously linked to aging or age-related disease.

Genetics of longevity in model organisms: debates and paradigm shifts

Discovering the biological basis of aging is one of the greatest remaining challenges for science. Work on the biology of aging has discovered a range of interventions and pathways that control aging rate. A picture is emerging of a signaling network that is sensitive to nutritional status and that controls growth, stress resistance, and aging. This network includes the insulin/IGF-1 and target of rapamycin (TOR) pathways and likely mediates the effects of dietary restriction on aging. Yet the biological processes upon which these pathways act to control life span remain unclear. A long-standing guiding assumption about aging is that it is caused by wear and tear, particularly damage at the molecular level. One view is that reactive oxygen species (ROS), including free radicals, generated as by-products of cellular metabolism, are a major contributor to this damage. Yet many recent tests of the oxidative damage theory have come up negative. Such tests have opened an exciting new phase in biogerontology in which fundamental assumptions about aging are being reexamined and revolutionary concepts are emerging. Among these concepts is the hyperfunction theory, which postulates that processes contributing to growth and reproduction run on in later life, leading to hypertrophic and hyperplastic pathologies. Here we reexamine central concepts about the nature of aging.

Enhanced energy metabolism contributes to the extended life span of calorie-restricted Caenorhabditis elegans

Caloric restriction (CR) markedly extends life span and improves the health of a broad number of species. Energy metabolism fundamentally contributes to the beneficial effects of CR, but the underlying mechanisms that are responsible for this effect remain enigmatic. A multidisciplinary approach that involves quantitative proteomics, immunochemistry, metabolic quantification, and life span analysis was used to determine how CR, which occurs in the Caenorhabditis elegans eat-2 mutants, modifies energy metabolism of the worm, and whether the observed modifications contribute to the CR-mediated physiological responses. A switch to fatty acid metabolism as an energy source and an enhanced rate of energy metabolism by eat-2 mutant nematodes were detected. Life span analyses validated the important role of these previously unknown alterations of energy metabolism in the CR-mediated longevity of nematodes. As observed in mice, the overexpression of the gene for the nematode analog of the cytosolic form of phosphoenolpyruvate carboxykinase caused a marked extension of the life span in C. elegans, presumably by enhancing energy metabolism via an altered rate of cataplerosis of tricarboxylic acid cycle anions. We conclude that an increase, not a decrease in fuel consumption, via an accelerated oxidation of fuels in the TCA cycle is involved in life span regulation; this mechanism may be conserved across phylogeny.

Icariin and its derivative icariside II extend healthspan via insulin/IGF-1 pathway in C. elegans

Compounds that delay aging might also postpone age-related diseases and extend healthspan in humans. Icariin is a flavonol extracted from several plant species of the Epimedium family. The icariin and its metabolic derivatives have been shown to exert wide protective effects in age-related diseases. However, whether icariin and its derivatives have the potency of delaying aging remains unclear. Here, we report that icariin and its derivative icariside II extend C. elegans lifespan. Using HPLC, we found high level of icariside II in the animals treated with icariin, suggesting icariside II is the bioactive form in vivo of icariin. Icariside II also increased the thermo and oxidative stress tolerance, slowed locomotion decline in late adulthood and delayed the onset of paralysis mediated by polyQ and Aβ_1–42 proteotoxicity. The lifespan extension effect of icariside II is dependent on the insulin/IGF-1 signaling (IIS) since the daf-16(mu86) and daf-2(e1370) failed to show any lifespan extension upon icariside II treatment. Consistently, icariside II treatment upregulates the expression of DAF-16 targets in the wild-type. Moreover, our data suggests that the heat shock transcription factor HSF-1 has a role in icariside II-dependent lifespan extension further implicating the IIS pathway. In conclusion, we demonstrate a novel natural compound, icariside II as the bioactive form of icariin, extends the healthspan via IIS pathway in C. elegans.

Solid plate-based dietary restriction in Caenorhabditis elegans

Reduction of food intake without malnutrition or starvation is known to increase lifespan and delay the onset of various age-related diseases in a wide range of species, including mammals. It also causes a decrease in body weight and fertility, as well as lower levels of plasma glucose, insulin, and IGF-1 in these animals. This treatment is often referred to as dietary restriction (DR) or caloric restriction (CR). The nematode Caenorhabditis elegans has emerged as an important model organism for studying the biology of aging. Both environmental and genetic manipulations have been used to model DR and have shown to extend lifespan in C. elegans. However, many of the reported DR studies in C. elegans were done by propagating animals in liquid media, while most of the genetic studies in the aging field were done on the standard solid agar in petri plates. Here we present a DR protocol using standard solid NGM agar-based plate with killed bacteria.

Hot topics in aging research: protein translation and TOR signaling, 2010

In this, the fourth installment of our annual Hot Topics review on mRNA translation and aging, we have decided to expand our scope to include recent findings related to the role of TOR signaling in aging. As new data emerge, it is clear that TOR signaling acts upstream of mRNA translation, as well as a variety of other cellular processes, to modulate longevity and healthspan in evolutionarily diverse species. This Hot Topics review will cover important new findings in this area that have occurred over the past year. These include the demonstration that the TOR substrate ribosomal S6 kinase modulates longevity in mammals, the potential for TOR inhibitors as therapeutic treatments for Alzheimer's disease, and further studies emphasizing the importance of differential translation of specific mRNAs for healthy aging and enhanced longevity.

TOR and ageing: a complex pathway for a complex process

Studies in invertebrate model organisms have led to a wealth of knowledge concerning the ageing process. But which of these discoveries will apply to ageing in humans? Recently, an assessment of the degree of conservation of ageing pathways between two of the leading invertebrate model organisms, Saccharomyces cerevisiae and Caenorhabditis elegans, was completed. The results (i) quantitatively indicated that pathways were conserved between evolutionarily disparate invertebrate species and (ii) emphasized the importance of the TOR kinase pathway in ageing. With recent findings that deletion of the mTOR substrate S6K1 or exposure of mice to the mTOR inhibitor rapamycin result in lifespan extension, mTOR signalling has become a major focus of ageing research. Here, we address downstream targets of mTOR signalling and their possible links to ageing. We also briefly cover other ageing genes identified by comparing worms and yeast, addressing the likelihood that their mammalian counterparts will affect longevity.